1. Field of the Invention
The present invention is related to automobile exhaust catalysts, and in particular, the present invention is related to thermally stable lean NOx traps.
2. Background Art
Environmental concerns and governmental regulations have been a continuing impetus for improvements in pollution control from automotive vehicles. The treatment or removal of noxious combustion by-products from the exhausts of such vehicles is a major focus of such efforts. Typically these combustion by-products include incomplete combustion by-products such as carbon monoxide and hydrocarbons. Moreover, the exhausts of these vehicles also include various nitrogen oxides. It is desirable and mandated that each of these compounds be reduced during vehicle operation.
Lean NOx traps are the technology of choice for treating the NOx emissions from lean-burn engines. The NOx traps are specially formulated with alkali metal or alkali-earth metals in the washcoat to enhance NOx storage capability. Under lean conditions, platinum in the washcoat oxidizes NO in the exhaust to NO2. The NO2 then reacts with the alkali-earth or alkali metals in the washcoat and some additional oxygen to form nitrates. Periodically, as the NOx storage capacity of the NOx trap is approached, the A/F ratio must be driven to a rich condition for a short time. Under these conditions, the nitrates decompose, releasing the stored NOx which then reacts with the reductants in the exhaust (HC, CO, and H2) over the precious metal to form N2. This purges the trap and regenerates the NOx storage capacity of the trap.
NOx traps are most effective at storing NOx under lean conditions in a temperature window that is typically in the approximate range of 250° to 450° C. At lower temperatures, the oxidation of NO to NO2 is slow and limits the storage performance. At higher temperatures, the thermodynamic stability of the nitrates decrease, limiting the amount of NOx that can be stored. At temperatures between 250° and 400° C., the NOx storage performance typically increases with increasing platinum loading due to an increase in the NO oxidation rate, particularly after high temperature aging. Above 400° C., the influence of the precious metal concentration decreases because the rate of NO oxidation increases. At these higher temperatures, the NOx storage performance is limited more by the amount of NOx storage sites, which is a function of the volume of the trap. This encourages the use of large NOx trap volumes with high platinum levels in order to provide good NOx storage performance at both low and high temperatures. However, this increases the cost of the aftertreatment system.
Due to the temperature sensitivity of the NOx storage performance, the NOx traps are typically placed in the underfloor location on vehicles. Small close-coupled catalysts are usually placed close to the exhaust manifold to provide fast light-off after a cold start. This configuration is beneficial for the durability of the NOx trap for two reasons. By being placed in the underfloor location, the NOx trap is not exposed to the high inlet temperatures that can occur at the exhaust manifold during high load operation. A second reason is that the engine is normally operated at stoichiometry during high load operation. Under these conditions, the HC, CO, and NOx in the exhaust are converted to CO2, H2O, and N2 at the precious metal sites. These reactions are exothermic and create local hot spots at the precious metal sites and the surrounding washcoat. This exotherm is highest in the front of the catalyst, where the concentrations of HC, CO, and NOx are highest, and drops as the concentrations decrease down the brick. This tends to age the front of the catalyst more than the rear of the catalyst. Therefore, having the NOx trap placed behind the close-coupled catalyst(s) is beneficial for the durability of the trap because the close-coupled catalysts will convert a high percentage of the HC, CO and NOx during high temperature operation, protecting the NOx trap from at least some of the exothermic reactions. However, even when aged behind a close-coupled three-way catalyst, the front zone of the NOx trap ages more than the back zone, possibly due to the conversion of HC, CO, and NOx that break through the three-way catalyst and react on the front zone of the NOx trap.
To reduce the cost of the aftertreatment system, there exists a need to reduce the precious metal loading in the NOx trap while maintaining good NOx conversion performance.